U.S. Pat. No. 3,592,286 describes a marine seismic survey system, known as the MAXIPULSE SYSTEM.RTM., which is based on the recognition that a useful seismic marine survey can be obtained with small explosives by correcting for or beneficially employing "bubble" pressure pulses. This is accomplished by obtaining a representation of the unreflected pressure waves in the body of water, created by the explosions of the small explosive charges, simultaneously with the conventional detection of the reflected acoustic waves, and suitably using both of the detected waves.
Upon detonation of each charge, the chemical energy in the charge is suddenly converted into kinetic energy of a rapidly expanding mass contained in a bubble. Because the gas bubble is normally generated at a depth of say 30 to 50 feet, it cannot vent directly to the atmosphere. This gas bubble therefore undergoes a very fast initial expansion which causes the surrounding water to become suddenly strongly compressed. Subsequent to the initial expansion, the bubble contracts then again expands then again contracts, etc. The entire sequence of such gas bubble expansions and contractions forms a high-pressure wave in the body of water which produces reflected, seismic, very-low pressure acoustic waves.
The marine seismic survey described in said U.S. Pat. No. 3,592,286 is predicated upon the ability to faithfully and substantially-linearly detect the unreflected, high-pressure wave created by each explosion.
Conventional detectors generally employed in the seismic industry cannot withstand such high-pressure waves which are like shock waves. The known seismic detectors comprise a fragile detector which converts acoustic energy into electric energy. More specifically, the detector produces a high-impedance charge or voltage when subjected to pressure. Thus, the detector acts as a pressure-to-voltage converter.
Since in a MAXIPULSE SYSTEM the detector need be positioned a distance of about 200 to 300 feet from the recording equipment, any resistance variations in the long line coupling the detector to the recording equipment may cause significant variations in the current impressed on the line by the high-impedance voltage from the detector. Accordingly, conventional seismic detectors would fail under the impact of the generated shock waves in the body of water, and their usefulness would be greatly reduced by the fact that such detectors act as high-impedance, voltage generators.
In sum, the detectors or hydrophones normally used in the seismic industry are required to detect very-low pressure acoustic waves, whereas the instantaneous shock wave resulting from an explosion may exceed 10,000 psi. There is therefore a need for a linear shock wave detector adapted for use in a MAXIPULSE SYSTEM in order to faithfully reproduce the high-pressure wave resulting from explosions of small seismic explosive charges.
A high-pressure detector probe is manufactured and sold by the PCB Piezotronics Corporation of Buffalo, N.Y. under several models, although the model of particular interest herein is Model 113A22. The detector probe is contained in an elongated metal casing housing a quartz crystal detector coupled to an amplifier which must be continuously energized. The detector probe is designed to be installed in the wall of a vessel the inside volume of which undergoes high-pressure fluidic variations. The end of the cylindrical probe which contains the input terminals to the probe is not exposed to and is protected from the shock waves, and only the other end of the cylindrical probe containing the quartz crystal is exposed to and communicates with the inside of the vessel.
By mounting the proble so that only the crystal side faces or is in direct fluid communication with the sea water, it was found that the probe's metal housing, which constitutes one terminal for the two-wire input, picks up electric noise signals from the sea water which are on the order of magnitude of the desired signals to be detected.
After considerable experimentation, I have found that by fully immersing the probe inside a dielectric fluid housed in a container having a flexible wall, not only does the probe not become damaged (as was generally believed that it would) but the metal housing of the probe becomes electrically isolated from the electric noise currents normally existing in the sea water and resulting from man-made objects and from natural phenomena. The container with the probe inside thereof will be hereinafter referred to as the detector assembly.
The location of the detector assembly on the gun relative to the charge launcher portion is governed by the ability of the probe to withstand the large shock waves generated in the ambient water by the explosions following the detonations of the charges and the ability of the probe to reproduce faithfully the pressure waveforms resulting from such explosions.
It is preferred to employ two such detector assemblies displaced from each other, so that in the event of a premature detonation which would take place too close to the first detector assembly, thereby affecting its output linearity, the output from the second detector assembly would provide a faithful reproduction of the pressure shock wave.